In the field access bubble memory device, a uniform rotating field is required for moving the bubbles around the circuit. For the packaged device, this field is normally generated by two orthogonally arranged flat solenoid coils. In the case when a whole wafer has to be tested before dicing into small devices, the flat solenoid approach is not applicable because it restricts both the electrical and mechanical access to the wafer. Therefore, an open coil structure which has no physical restriction on the wafer plane is required.
There are numerous applications needing a uniform field in an open space, such as bubble wafer probings, wafer visual inspection, and testing for a bubble chip bounded in a chip carrier. Prior to the present invention there has not been a small, efficient coil configuration to meet such requirements.